There are many applications for using microwave energy to heat objects. Microwave ovens are commonly used to heat food. There are also many scientific and industrial applications of microwave heating.
Microwave heating takes place in two basic processes depending on the object's properties. Where the object to be heated is a dielectric, microwave power is dissipated within the nonconducting medium through a volumetric heating process. Where the object is a conductor, microwave heating takes place within a small surface penetration depth which depends on the dielection properties of the material and the microwave frequency.
Microwave heating has a number of advantages over other techniques. The air and other materials around the object are generally not heated as they would be in conventional ovens. This reduces wasted energy. Moreover, conventional ovens apply heat to the outside of an object, relying on conduction from the outside surface to heat the object's interior. The result is less heating deep inside the object than near the surface. A number of problems are caused by this non-uniform application of heat. These include cracking from heat gradients, and scorching or burning of the surface of the object. In contrast, microwaves can heat the interior and exterior of an object in a relatively uniform manner.
Despite these advantages, microwave heating has some disadvantages. Heating is uniform only under special uniform conditions that depend on the modes excited in the oven as well as the composition, shape, size and location of the sample in the oven. Non-uniform fields will result in non-uniform heating. Hence some areas inside an object will be heated more than others; or two different objects will be heated by different amounts. This is due to two primary factors: microwave fields are inherently non-uniform within the oven, and objects within the oven will distort the fields near the object. While the inherent non-uniformities are predictable, distortions from objects within are difficult to predict and quantify. This is because these distortions, or perturbations, depend on the dielectric properties and shapes of the objects in complex ways.
Non-uniform heating limits the usefulness of microwaves in both consumer and industrial applications. In consumer applications, the result is often overcooking and/or undercooking of portions of food in the oven. In some industrial applications, such as batch processing of materials and products at high temperatures, non-uniform heating results in non-uniform products. This occurs when the desired processing depends on the samples receiving a precise amount of heat.
There have been numerous attempts to overcome the problem of non-uniform microwave heating. In one technique, objects are moved within the oven during heating to expose different objects, and different parts of each object, to varying microwave energy levels. Another method is to excite a large number of resonant modes within the microwave oven. This can be done, for example, by introducing a moving deflector into the microwave field. These approaches often do not achieve a satisfactory level of uniformity, particularly in high temperature industrial applications.
The inventors of the present invention have devised a new way to address the problem of non-uniform heating in microwave ovens. The present invention provides a way to configure a microwave oven so that the resonant mode of the microwaves, and the particular physical characteristics of the oven and samples, result in substantially identical field configurations surrounding all the samples. This results in the same processing conditions for each sample. The primary obstacle preventing uniform batch processing has been the complex perturbations in microwave fields created by the heated object's shape and dielectric properties. These perturbations would have to be quantified for each spatial location in order to account for these variations. For most applications, these distortions are seemingly intractable to quantify.
The inventors of the present invention have recognized that a fundamental understanding of the microwave/sample interactions could result in the prediction of a set of experimental parameters that would lead to uniform batch processing.
The present invention arises from the inventors' discovery of a way to take advantage of the symmetry of certain microwave and sample configurations that can produce uniform heating of multiple samples.
In a preferred embodiment of the invention the microwave mode of excitation and cavity shape are configured so as to generate predictable regions of identical microwave field strength. Samples placed in these regions will generally receive equal amounts of microwave energy and heating. However, larger samples that are close to each other introduce additional perturbations in the microwave fields of the other samples in the oven. In general, microwave fields at other sample locations are significantly affected by these disturbances. In the present invention, the perturbations induced by adjacent samples facilitate instead of destroy uniform processing conditions.
According to one aspect of the invention, a microwave cavity is configured to have known regions where the microwave field patterns would be almost identical without the samples in the cavity. This requires the selection of a particular cavity shape and microwave resonant mode. Then the samples are placed in specific geometrically symmetrical locations. This particular arrangement produces a modified microwave field configuration due to the sample-induced perturbations at every sample location. In this carefully determined configuration, symmetry in the perturbations caused by all of the samples will be identical for each sample location. This eliminates the need to calculate and predict the exact perturbations caused by each sample. Each sample will be heated in a similar manner under these conditions.
The present invention uses a fundamental understanding of the microwave/material interactions to produce uniform batch processing conditions employing techniques which are not taught or suggested by the prior art. According to the present invention, the sample-induced perturbations are not necessarily quantified, but nevertheless are used to create identical perturbations at each sample location. A number of new applications requiring uniform batch processing of materials are now possible using the invention. These applications may now take advantage of the many benefits of microwave heating over conventional heating processes.